If iSCSI or Fibre Channel SAN disks are properly connected/configured but still not available in Windows Disk Management, the SAN policy may need to be turned on in Windows. To check/change this:
Open an elevated command prompt and type
san, and it should show the SAN policy.
If it says "SAN policy : Offline All", then type
You may still need to manually bring the disks online the first time in Disk Management (see Windows Server screenshot), but they should stay online on subsequent reboots because of the policy change.